1) Field of the Invention
The present invention relates to an image display apparatus including a current-controlled light emitting diode and a driver element that restricts a current value flowing into the current-controlled light emitting diode, and more specifically, relates to an image display apparatus that can write, a voltage including a threshold voltage fluctuation in the driver element, without using a special-purpose current source.
2) Description of the Related Art
An organic light emitting diode (hereinafter, “organic LED”) display apparatus using an organic electroluminescent (EL) device that emits light itself, is most suitable for making the apparatus thin, since it does not require a backlight, which is required in a liquid crystal display apparatus, and does not have any limitation in the angle of visibility. Therefore, practical use thereof is expected as a next-generation display apparatus, which takes the place of the liquid crystal display apparatus.
As the image display apparatus using the organic LEDs, a simple (passive) matrix type and an active matrix type are known. The former has a simple configuration, but has a problem in that realization of a large-scale and highly delicate display is difficult. Therefore, development of the active matrix type display apparatus has been recently performed, which controls the current flowing into light emitting diodes in pixels, by an active element provided in the pixel, for example, a driver element including a thin film transistor.
Such a driver element is connected in series to the organic LED, and at the time of image display, a current equal to the current flowing into the organic LED flows to the driver element continuously. Therefore, when the image display apparatus is used over a long period of time, the electric characteristics of the driver element considerably deteriorates, causing a problem of, for example, fluctuations in the threshold voltage. When the electric characteristics of the driver element considerably deteriorates, a current having a value different from an intended value flows in the organic LED, and hence the luminance of the light emitted from the organic LED fluctuates, thereby deteriorating the quality of the displayed image.
Therefore, an image display apparatus having a compensation circuit for compensating the fluctuations in the electric characteristics of the driver element has been proposed. FIG. 10 is a circuit diagram depicting one example of the configuration of the image display apparatus having the compensation circuit. As shown in FIG. 10, the conventional image display apparatus includes a select line 210, and a data line 220 connected to a current source 230, and further includes p-type transistors 240, 250, and 260, which are connected to each other, and to the select line 210 and the data line 220, an n-type transistor 270, a capacitor 280, and an organic LED 290. The p-type transistor 260 serves as a driver element, and the capacitor 280 is connected between the gate and the source of the driver element. Therefore, the voltage applied to the capacitor 280 becomes the gate to source voltage of the p-type transistor 260, being the driver element, and the current value flowing in the p-type transistor 260 is determined based on the gate to source voltage.
The process for supplying potential to the capacitor 280 will be explained. At first, when the potential of the select line 210 becomes low, the p-type transistors 240 and 250 are turned on, so that the gate and the drain of the p-type transistor 260 become conductive to each other, and the data line 220 and the source electrode of the p-type transistor 260 become conductive to each other. It is assumed that the current source 230 connected to the data line 220 supplies electric current having a value corresponding to the display luminance, and the current is supplied to the p-type transistor 260 via the data line and the p-type transistor 250.
The gate electrode and the drain electrode of the p-type transistor 260 have the same potential, since the p-type transistor 240 is in the ON state, and hence the gate to source voltage corresponding to the current value supplied from the current source 230 is generated in the p-type transistor 260. Since the capacitor 280 is arranged between the gate electrode and the source electrode of the p-type transistor 260, a voltage corresponding to the gate to source voltage provided at this time is accumulated in the capacitor 280, thereby finishing the voltage write with respect to the capacitor 280. The voltage written in the capacitor 280 becomes the gate to source voltage of the p-type transistor 260, being the driver element, and at the time of light emission, current corresponding to such a voltage flows into the organic LED 290, thereby performing light emission.
Thus, the gate to source voltage of the p-type transistor 260 is determined based on the current actually flowing between the source and the drain. Therefore, even when fluctuations in the threshold voltage occur, the gate to source voltage including such fluctuations is determined. As a result, current having a desired value can be made to flow into the organic LED 290, regardless of deterioration in the p-type transistor 260 (See U.S. Pat. No. 6,229,506 for example).
In the circuit shown in FIG. 10, however, there is a problem in that long time is required for writing voltage in the capacitor 280. That is, in the configuration shown in FIG. 10, at a voltage writing phase, the current from the current source 230 is supplied to the p-type transistor 260 through the data line 220 and other wiring structures. Therefore, predetermined time is required until the current flowing into the p-type transistor 260 reaches a predetermined value, resulting from the parasitic capacitance included in the data line 220 and the like, and as a result, the time required for voltage write increases.